Tumor resistance to radiotherapy remains a significant barrier to improving outcomes of patients diagnosed with locally advanced, unresectable cancers. To overcome radioresistance, drugs that sensitize tumor cells to ionizing radiation (IR) are used. In theory, more potent radiosensitizers should increase tumor kill and improve patient outcomes. In practice, the clinical utility of more potent radiosensitizing drugs i curtailed by dose limiting systemic side effects and radiosensitizing normal tissue in close proximity to the irradiated tumor. To provide a solution to these clinical problems, we are developing and testing antibody drug conjugates (ADC) to selectively deliver potent radiosensitizers to tumors based on tumor cell surface receptor expression. ADC consist of a drug covalently attached to an antibody recognizing a specific cell surface receptor. ADC binds to cells expressing the receptor, then internalized by receptor-mediated endocytosis, and finally the drug is released from the antibody by endolysosomal proteases. To develop our ADC radiosensitization strategy in a clinically meaningful manner, we have focused on using the therapeutic antibodies cetuximab and trastuzumab that bind ErbB family members EGFR (ErbB1) and HER2 (ErbB2) respectively. As therapeutic antibodies blocking ErbB signaling, both cetuximab and trastuzumab have a therapeutic window and are clinically used in cancer patients in combination with radiotherapy. However, the overall efficacy of such signal inhibition strategies is limited due to the emergence of bypass signaling pathways that unfortunately result in therapy resistance. The goal of our proposal is to widen the therapeutic window of cetuximab and trastuzumab, by re- purposing their therapeutic task. Instead of being utilized as inhibitors of ErbB signaling, we propose they be used as receptor targeted delivery vehicles for potent radiosensitizing drugs. In Aim 1, we will synthesize novel trastuzumab/cetuximab ADC and test if they radiosensitize tumor cells in a HER2/EGFR receptor restricted manner. We will test two classes of radiosensitizers, broadly cytotoxic cell cycle poisons and targeted inhibitors of DNA damage repair. Since HER2 receptor expression is tumor selective, we hypothesize trastuzumab conjugated to highly potent anti-tubulin auristatin will restrict its tumoricidal activty and radiosensitization to HER2+ tumor cells. In contrast to HER2, inhibiting EGFR results in toxicity to skin and gastrointestinal tract, precluding conjugation of cell cycle poisons. Therefor, we hypothesize cetuximab conjugated to drugs that inhibit the DNA damage response will selectively radiosensitize EGFR+ tumors. In Aim 2, we will test our most potent radiosensitizing ADC in murine tumor models. ADC delivery of radiosensitizers has multiple innovative features. First, it is a biomarker-based radiosensitization approach that is histology agnostic. Second, it allows for the clinical testing of more potent radiosensitizers since drug delivery is restricted. Finally, it alters the paradigm for ErbB radiosensitization, using tumor cell surface receptors as beacons for potent drug delivery, instead of as targets for signal inhibition.